The following is a sidebar to an article that describes the evolution of grain size measurement and ASTM stardard E 112. It was written by George Vander Voort on the occasion
of the 75th anniversary of Committee E-4 on metallography and
originally appeared in ASTM Standardization News, May, 1991 as "Committee
E-4 and Grain Size Measurements: 75 years of progress." It is reproduced
here with the kind permission of Mr. Philip Lively, assistant VP for
publications, marketing, and information technology at ASTM.

GRAlN SlZE DlSTRlBUTlONS

The previous examples of grain structures were of equiaxed grains with
a uniform size distribution, that is, if a frequency histogram is plotted
of the grain diameters, intercept lengths, or areas, a single peak is observed
which will be symmetrical in shape if the dimension scale is logarithmic.
However, there are occasions where more complex distributions are observed,
often in specimens where recrystallization is incomplete, or at the onset
of rapid grain growth. Test Methods E 112 is not well suited for such
specimens--Methods E 930 or E 1181 must be used, usually the latter.

As an illustration, Figure A (the figures for this sidebar are in the
margins) shows the microstructure of an experimental hot work tool steel
that was austenitized at 1975 degrees F ; grain growth was very rapid at
slightly higher temperatures. Etching revealed the prior-austenite grain
boundaries which, except for one very large grain, are from a single
distribution. In measuring the grain size of this specimen, we could determine
the grain size of this one very large "rogue" grain by Methods E 930 and
the rest oi the grains using Test Methods E 112. A somewhat similar example
is given in Figure B, which shows the onset of grain growth in A286. These
are twinned austenite grains revealed by a grain contrast etch, rather than
a flat etch as in Figure A. We could measure the largest of these grains
by Methods E 930 and the fine region by Test Methods E 112, or we could use
the approaches in Methods E 1181 for the entire structure.

Sometimes the duplex nature of the grain size distribution is highly
segregated, in other cases the two distritbutions are intermixed. Figure
C shows a relatively well mixed bimodal distribution of grain sizes. This
is a superalloy specimen with a flat etch containing annnealing twins. Figure
D, on the other hand, shows an extremely segregated example of a duplex
condition. Note that grain growth has occurred at two locations along the
surface of this low-carbon steel specimen. Another segregated form of a duplex
condition is the so-called "necklace" type, as shown in Figures E and F.
Both are highly alloyed stainless steels that have not been fully recrystallized.
The fine recrystallized grains surround the large non-recrystallized grains.
The main difference between the two examples is the pronounced elongation
of the non-recrystallized grains in Figure F due to the difference in sectioning
plane orientation, transverse vs. longitudinal, for the two.

The amounts of the fine and coarse grains can vary considerably. Obviously,
the percentage of the number of grains of each type will be much different
than the area or volume percentage of each type. Figure G shows a more equal
area percentage of fine and coarse grains in a nickel-base superalloy. In
some cases, the different grain size distributions may be in a layered or
banded manner due to the influence of segregation. Figure H shows such a
pattern in ferritic stainless steel plate specimen. Note the very long, thin
non-recrystallized grain.

The grain size of such specimens should not be described by a single
average value, as it is quite possible that there will not be any actual
grains of that size in the specimen; or, the average chosen may be in one
of the tails of the two grain size distributions. The best approach is to
determine the area percentage of each grain size distribution and the average
grain size of each distribution in specimens with a duplex or bimodal grain
size distribution, as described in Methods E 1181.